Ectothermic and Endothermic reactions

[Caleb]

Is there a way to tell if a reaction will be ectothermic or endothermic just by looking at the chemical equation? Thanks

[Horza2002]

Yes and no....

 

On the whole, most reactions are exothermic.

 

But in an endothermic reaction, the bonds in the product are weaker than they are in the reactants. To overcome this, there has to be an increase in entropy. So if a reaction produces a lot of gas molecules then its possible it'l be endothermic.

[Caleb]

So if a reaction produces a lot of gas molecules then its possible it'l be endothermic.

 

What about gasoline and oxygen? With plenty of oxygen, this reaction makes nothing but CO2 and H2O. Still it is a very ectothermic reaction.

[Horza2002]

Well CO2 is a gas so it will be favoured anyway....but you have to consider the bond strengths....C=O in the carbon dioxide are very very strong. Like I said....enthalpy is not a good indication of whether a reaction will work or not

[Caleb]

Thanks.

[Horza2002]

No problem. If you want me to explain it in more detail just let me know!!